Micro-electro-mechanical transducers usually share a common feature which includes a movable mechanical part used for energy transformation. One example of such micro-electro-mechanical transducers is micromachined ultrasonic transducers (MUT). An ultrasound transducer performs a chain of energy transformation to realize its function of a transducer. In its receiving mode, the acoustic energy of ultrasound waves propagating in a medium where the transducer is placed is transformed to mechanical energy of a movable part (conventionally a vibrating membrane) in the transducer. The motion of the movable part is then transformed to a detectable electromagnetic (usually electrical) signal. In its transmitter mode, the reverse chain of energy transformation takes place.
Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. FIG. 1 shows a cross-sectional view of a basic structure of a prior art cMUT. The cMUT 10 of FIG. 1 is built on a substrate 11. Each cMUT cell has a parallel plate capacitor consisting of a rigid bottom electrode 12 and a top electrode 14 residing on or within a flexible membrane 16 that is used to transmit or receive an acoustic wave in the adjacent medium. The flexible membrane 16 in each cell is supported by the anchor 18. The membrane 16 is spaced from the substrate 11 and the top electrode 12 to define a transducing space 19 therebetween. A DC bias voltage is applied between the electrodes 12 and 14 to deflect the membrane 16 to an optimal position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane 16 in order to deliver acoustic energy into the medium (not shown) surrounding the cMUT 10. During reception the impinging acoustic wave vibrates the membrane 16, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change.
The two electrodes of the cMUT are usually desired to be parallel during operation to achieve optimum performance. Ideally, the top and bottom electrodes may both be rigid (that is, the deflection of both electrodes is much smaller than the change of the separation distance between two electrodes during the operation). However, in the cMUTs reported so far, at least part of one or both electrodes is made of flexible structures (e.g., a flexible membrane, cantilever, spring, etc.), so the dynamic status of the two electrodes during operation may not be parallel even if the two electrodes are designed to be substantially parallel to each other when static.
In addition, unlike PZT transducer, the electrostatic force in cMUT is not linearly proportional to the applied voltage and the electrode separation. This nonlinearity of the electrostatic actuation may degrade the transducer's performance and reliability.
Due to the importance of these MUT devices, it is desirable to improve the technology in terms of performance, functionality, and manufacturability in general, and to optimize transduction performance, breakdown voltage and parasitic capacitance reduction in particular. In order to increase the average electrical intensity and to enhance reliability, the shapes of the internal surfaces, such as the profile of the separation gap between two cMUT electrodes and the spring-substrate contact areas, may need to be optimized for a cMUT. This optimization is especially desired for correcting non-parallel motion between two electrodes and enhancing breakdown (collapse) voltage in the cMUT. Furthermore, new methods of fabrication are decided because designs having a special shaped surface may be difficult to fabricate using a conventional fabrication process given the very small separation between the cMUT electrodes.